Knocking generated in an internal combustion engine causes reduction of engine output or damage due to overheating of the engine. The maximum engine output can be achieved by advancing the ignition timing to a value just before the knock occurs in the engine characteristics. If the ignition timing is further advanced the knock will occur, and if retarded further the engine output will be reduced.
It is desirable, therefore, to maintain the ignition timing at a value just before the knock occurs.
In such an ignition timing control apparatus, it is necessary to accurately judge the knock signal.
When the engine is used for a long time there is produced a variation in the clearance of the intake and exhaust valves, so that the intensity of the vibration of the engine is changed, which frequently results in an incorrect detection of the knock signal. For this reason, a checking device is required to determine whether the knock detection device is detecting proper knocking of the engine.
Examples of checking devices of this type are disclosed in Japanese Patent Laid-Open Publication Nos. 57-68559 (1982) "Knocking diagnosis apparatus" published on Apr. 26, 1982, and U.S. Pat. No. 4,425,891 "Engine knock control apparatus" published on Jan. 17, 1984.
However, those checking devices disclosed in the publications mentioned above merely judge whether the checking devices are operating or not.
Sensors used for detecting knocking are sometimes not uniform in their output characteristics, and the output characteristics differ from the original characteristics depending on the conditions of mounting the sensors on the engines. Furthermore, the characteristics change due to deterioration with age.
For this reason, in order to judge not only whether the detection device is merely operating or not, but whether the accurate ignition timing control is achieved or not, it is necessary to check whether the detection sensitivity is appropriate or not.